A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a time lapse (often referred to as a 4-D) survey is one that is taken over the same subsurface target at two or more different times. This might be done for many reasons but often it is done to measure changes in subsurface reflectivity over time which might be caused by, for example, the progress of a fire flood, movement of a gas oil or oil/water contact, etc. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) may be attributable to the progress of the subsurface processes that is at work.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2-D survey, there may usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2-D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3-D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 milliseconds and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profiles) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
The data in a 3-D survey are amenable to viewing in a number of different ways. First, horizontal “constant time slices” may be extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2-D plane of seismic data. By animating a series of 2-D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2-D seismic line from within the 3-D data volume. It should also be noted that a 3-D dataset can be thought of as being made up of a 5-D data set that has been reduced in dimensionality by stacking it into a 3-D image. The dimensions are typically time (or depth “z”), “x” (e.g., North-South), “y” (e.g., East-West), source-receiver offset in the x direction, and source-receiver offset in the y direction. While the examples here may focus on the 2-D and 3-D cases, the extension of the process to four or five dimensions is straightforward.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
Of course, one well-known problem with seismic data is that it is can be expensive to acquire. Indeed, in some cases the cost of the survey may determine whether or not the economics of the proposed target are favorable. Thus, techniques that tend to reduce the cost of such surveys are always welcome.
Spaced firing of two or more sources has been recognized as one strategy for reducing the cost of seismic data acquisition. The basic idea behind this approach is that a receiver line or patch may be deployed and that multiple sources may be sequentially or simultaneously activated during a single recording period. Thus, subsurface reflections from an early source excitation may be comingled with those that have been sourced later, i.e., a “blended source” survey is acquired. Note that this is in stark contrast to conventional surveying techniques, wherein the returning subsurface reflections from one source would never be allowed to overlap the reflections of another.
Although the blended source approach has the potential to dramatically reduce the time in the field, thereby reducing the cost of the survey proportionally, one obvious problem is that it can be difficult to separate the individual shots thereafter. Said another way, what is of critical importance in interpreting seismic data is the depth of each reflector. Generally speaking, depth of a reflector is determined by reference to its two-way seismic travel time. So, in a multiple source survey it is of the highest priority to determine which of the observed subsurface reflections is associated with each source, otherwise its two-wave travel time cannot be reliably determined.
In addition to planned blended source surveys, in some cases unplanned multiple source recordings may be acquired. For example, in areas of intense exploration activity there may be several crews shooting in the same general area. This may be of particular concern in marine areas where multiple seismic boats may be active simultaneously. Traditionally when a seismic record contains energy from a third party source some attempt is made to mute the part of the signal that contains the unwanted source so that it does not spread to adjacent records via multi-trace processing algorithms such as migration. However, such muting eliminates both the interfering noise and useful reflections that might occur at or near the same reflection time. Although it is known to try to replace the muted regions by interpolation from unmuted data, such is at best an approximation to the data that have been lost.
Separating the two or more shots from a single seismic recording has been predictably problematic. Shot scheduling during acquisition has shown some promise in terms of improving the separability of simultaneously acquired shots, and this has been especially so where the time interval between successive shots is at least somewhat random. However, this approach has not proved to be as useful, at least in part because heretofore there has not been a reliable way to determine how much “randomness” in the time separations would produce an optimal (or even a good) result. Further refinements of this approach are clearly necessary.
Heretofore, as is well known in the seismic acquisition, processing, and interpretation arts, there has been a need for a method of separating two or more seismic sources that have been activated during a single recording. Accordingly, it should now be recognized, there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.